Outward burning neutral granulation for cast propellants



Aug 6 1963 s. DOBRIN ETAL 3,099,963

OUTWARD BURNING NEUTRAL GRANULATION FOR CAST PROPELLANTS Filed Dec. l1, 1950 5 Sheets-Sheet l Jul-m F Kincaid m /wwfm woz/W9 United States Patent O 3,099,963 OUTWARD BURNING NEUTRAL GRANULATHGN FOR CAST PRUPELLANTS Saxe Dohrin, Union, and John F. Kincaid, Moorestown,

NJ., assignors to the United States of America as represented by the Secretary 'of the Army Filed Dec. 11, 1950, Ser. No. 200,256 13 Claims. (Cl. 102-93) This invention comprises a new and improved outward burning neutral granulation for cast propellants.

It is well known that the shape of an explosive propellant grain has a considerable effect on its burning rate. Rodman, about the middle of the nineteenth century, developed the perforated cylindrical grain burning partially from the inside out. Since his time single or multiperforated grains have been standard. `Gathmann in Patent 660,568 discloses several such grains. Walsh in Patent 11,077,320 shows another example of the multiperforated type, the rosette, burning more nearly completely than the ordinary multiperforate powder. The burning rate can also be controlled by means of restrictive coatings placed on selected portions of the surface. These coatings prevent burning of the areas covered.

The evolution of gas and hence the propulsive force of the propellant is dependent on .the burning rate. -A solid grain of any shape will burn with a continuously decreasing surface and burning rate with consequent loss of pressure. This is known 'as degressive burning. A single perforation such as Rodman first used will allow burning from the inside out. The internal area in this case will increase at just about the same rate as the external will decrease. Constant evolution and pressure of gas results and the grain and burning are termed neutral. Progressive burning occurs when multiple peforations `are employed, that is, the exposed surface will continually increase with increased evolution of gas and pressure. Restrictive coatings change the burning rate in obvious fashion. If for example a singly perforated cylindrical stick or grain is entirely covered externally, the internal surface, continually increasing, yields progressive instead of neutral burning.

We have discovered that desirable burning properties can be incorporated by the use of slots cut into the body of the propellant grain. It is therefore an object of this invention to produce a new and improved slotted granulation for powder. More specifically, we have `found that a neutral propellant can be formed from an externally covered perforated cylindrical grain by the use of radial slots cut in one end of the grain. lt is consequently another object of the invention to use radial slots to produce a neutral grain from one restrictively coated externally. Other objects will appear hereinafter.

-In the drawings:

`FIG. l shows a top view of a grain of powder with six radial slots forming equally spaced sectors;

FIG. 2 represents a vertical section of the grain taken along the line 2--2 of FIG. l;

FIG. 3 shows a top view of a grain with tive radial slots forming unequal sectors;

FIG. 4 shows the variance of the surface of an uncompensated single perforated grain restricted on the sides and one end, the degressive effect of the end area becoming less with increase in grain length;

FIG. 5 shows the rate at which the surfaces of various sectors will decrease;

YFIG. 6 shows the burning of one sector, B representing the central angle;

PIG. 7 shows the burning of a grain with ve equal sectors;

ICC

FIG. 8 shows the burning of a grain with unequal sectors;

FIG. 9 shows the geometric relationship of burning sur-face to distance burned; i

FIG. l0 shows U-shaped burning at the bottom of a perforation.

These gures show a cylindrical grain stick or rod of powder 1 with a longitudinal perforation Z extending throughout its length and a coating 3` restricting burning externally except alo-ng the top surface. In one ernbodiment of the invention equal sectors 4 are formed by slots 10 cut into the exposed top and extending about one fourth of the distance between the ends of the grain. In another embodiment unequal sectors `5-9 are formed by the slots 10 extending about one-fourth the iength of the grain as. in the previous case.

Upon ignition the stick burns `on top, from the perforation out and along the slots. The central area of the perforation constantly increases but the area of the sectors defined by the slots continually decreases. The rates of change in these areas can be made to cancel each other and thus produce a neutral propellant, the regressiveness of the slots compensating for the progressiveness of the perforations. As the length of the grain increases the degressive effect of the circular end area becomes less.

'Ilhe initial surface is increased by cutting; the slots lin the end `of the grain, the rnumber and depth of the slots depending on the amount :of surface necessary. As noted above, the surface exposed by the slots becomes less while the burning progresses. The rate at which the surface of the slots Iwill decrease depends upon the central angles of the sectors, that is, the angle between slots. i

If the slots are cut at equal angles as in FIG. 1 the compensating effect will disappear midway through burning, FIG. 7. To avoid this usually undesirable resuit, the sectors should be designed las in FIG. 3 with different central angles and will be consumed, one by one, over a predetenmined interval off time.

Several advantages are offered by the geometry of the present invention. For example, the pressure-time record is more nearly neutral rwith this grain than it is with the competitive star-centered or cone and cylinder types. Strict neutnaiity is especially desirable where constant thrust propellants are necessary as in jet assisted take off units.

Furthermore, the instant granulation oifers a long burning distance. The burning distance is the complete length of web yof the perfonated part. For a grain with an 8.5 inch outside diameter and a 0.5 inch internal diameter the burning distance is 4 inches. The longer the burning distance the longer the period of gas evolution and hence of thrust.

Granulation of the slotted type is extremely simple to manufacture. 'I'he desired shape can be made by placing a non-adhesive core in a restrictive container and fill-ing the volume with cast propellant. Put together in this manner, the unit possesses Ia very high loading density. Ninety-eight percent of the volume of the container can be iilled with the propellant and its restrictive coating, ninety percent by the propellant itself.

Advantages in economy of material-s and. design of the container are also found. Even in the rosette grain some small slivers of powder remain unburned although the Walsh improvement reduces such losses by ii-fty percent over the usual perforated grain. Use of the slots, bowever, eliminates un'bu-rned materials. Design of the container is `simplified because of the ease of gas elimina- .tion where effectively a single perforation is used. No difficulty is consequently encountered in either por-ting 3 or -trapping arrangements. Gases produced by slow decomposition also escape without difficulty since the diffusion distance vfor such gases is never very great.

The size and dept-h of the slots and sector angles may be calculated :from'tthe known characteristics of the propellant used. As an illustration, the design of a neutral out- Ward burning charge for an eight second jet assisted .takeolf unit will b-e given.

For the purposes of calculation it IWill be assumed that The minimum burn-ing distance of the grain, the web, is:

8 sec. 0.5 in./sec..=4 in.

.Allowing `0.5 inch Ifor a restrictive coating, the external diameter is 8.5.inches. A ,perforation lwith a diameter of 0.5 inch leaves a web of 4.0 inches.

If a minimum thrust of 9,00 lbs. at 40 is chosen, the rate `of discharge is:

F 900 m---LG 1b./see. and lthe burning time tb 4 m 10.67 sec.

:tm The total Weight of propellant required is therefore: m tb=10.67 4.66=49.9 1b. V(50 1b.)

The surface required is:

At the upper temperature limit this dimension will provide a thrust of 'The powder Volume is 50 M5893 eu. 1n. 'Ihe length of the grain will be approximately:

893 l-7r `/4[(85)2 (umn- 15.8 1n. The length of the grain `at the inal instant of burning will be Theltlength of vgrain which Iwill be consumed by end burning will be equal to the web, 4 inches. Thus the depth off-the slots -must be:

1:5.80f8.33-4.0=3.47 in. Arbitrarily the width of the slots is set as inch. This `Width `will proyide more than enough port area.

To simplify calculations, a hypothetical grain rshape 4 will be assumed which will be consumed exactly -as the actual grain with the regressing sectors. This hypothetical grain will have Ithe yfollowing dimensions:

O. D 8.5 in.

I. D 0.500-2(%2)=0.312 in. Length 15.80%2=.15.89 in.

Web 4.09 in.

Width of slot 0.00 in.

After all surfaces ofthe hypothetical grain have Iregresdsed 3/{32 inch, the hypothetical grain will Ihave the same shape as the actual grain.

At ythe beginning of burning the slots must provide' Each slot provides 2 4.0\9 3.50=28.6 s q. in. .and

slots are required. 0f ycourse iive slots will be cut, each exposing 30 sq. in. the depth being The overall length of the hypothetical gra-in will be 3.67-l-8.33{4.09=16.09 in.

The dimensions now established are:

Hypothetical Actual 0.D., in-- s. 5 8. 5 I.D., in. 0.312 0.5 Length, 1n 16.09 16.00 Number of slots 5.0 5. 0 Depth of slots, in 3. 67 3. 67

For any sector the distance of burning is Sector- Angle, 6 25 During burning the side surface of a sector will decrease las the parallel layers .meet each other at the point'.

'Ilhe length c of one side of a sector after a distance p has been burned is c=\/R2-p2-p ctn a (FIG. 9) Because of the perforation c=4.09 in. R=radius of grain/2.

Ilhe total surface exposed on the faces of the sectors at any time is: Ss=2 (depth of slot) 2c Where 2c=\/R2p2-p` ctn 12.50

It has been yassumed that the bottoms of the slots burn with square corners. Actually the corners burn U-shaped, as shown in FIG. 10. The decrease in surface due to this effect is AS=2.[2p-% 21rp]2c=0.86p2c Symbols here are .as shown in FIG. 10.

The slots in 'the actual 'grain will be cut with rectangular bottoms for which -AS=0.86(p-0.09)2c 3f deeper slots is tabulated in Table 2 and shown graphically in FIG. 8.

TABLE 2 Burning Surface of Slotted Grain Surface End and U-Shape Correction Total P 2C,n.2 0f sectors, lateral surcorrection, for "Runt surface,in.2

in.2 face 111.2 in.2 sector 111.2

Table I is the tabulation of all lche surface at equal 25 The variation in surface during burning is only intervals of burning distance, using the dimensions calculated. From Table I -it can' be seen -that the calculated 2268-2191 X100=3 57 shape will result in vfairly smooth burning. But at 223 o p=1.25 inches the surface is 10.4 square inches, 5% below the desired 222 square inches. To supply the extra v Scale model grams; have be@ fed- Slots were mlued surface the Slots Could be Cut 1n 2.75 X 0.75 X 5.75 1n. composite propellant grains. The variation in surface during burning was expected to be loi-:Q51 higher than calculated, due to rough machining, but 1008 amounted to only 13%. in ydeeper (by increasing total length). However, at What We OlaimSI p=0 the increase in surface Would be 22 square inches 1- All Out-Walid bUIllIlg nel-11ml .gfIl-Ula'flol 'for C'ast and at p=0.25, 19.1 square inches. This overbalance PTOPeH'aUS C011S1S't111g 0f a Sfhd Cylmdl'lcal .gram hal/111g at the beginning can' be corrected by making the first sectop and bottom Surfaces. a Side Wall and a length Igreater ter e runt, either by cutting it en inch er two lower than than the radlus of Said erm. Sald 8ra1n a1s0 havmg a the rest or by recessing the point from the center. Thus 40 longltudmal aXlS 0f Symmetry and all 3X1a1-11ff0rat1011 the .dimensions of the grain are; coextensive with said depth, a restrictive coatmg covering said side Wall and said bottom surface, there being a plurality of angularly related slots coextending radially Hypothetlca] Actual outwardly between said perforation and the outer curved O D 8 5 8 5 45 surface of said grain, said slots having an axial dimen- Ijyjilrfljf 0,3'12 015 sion smaller than said length and equal approximately to Web. 409 4-0 said radius, said slots opening through said top surface. Ilnfonf'scg m5?, 1mg 2. The propellant of claim 1, there being six slots Depth UfSlOtSiflS 4- 17 4- 17 formed in said grain, said slots being equally angularly related.

TABLE 1 [Surface of hypothetical grain 8.5 x .312 x 16.0, 5 slots 0.00 inches wide by 3.67 inches deep cut at 25, 50, 75, 100, 110] Distance Annular Surface 0f Surface of Correction Total snrburned, in. end surface, perforation, c, in.2 slots, in.2 for U-shape, face, in! in.2 in.2 in.2

The 'angles of the sectors are to be 25, 50, 75, 100, and A 2.0 inch layer is to be cut from the top 3. The propellant of claim 1, there being iive slots formed in said grain, said slots being unequally angularly of the 25 sector. The surface of the grain With the 75 related.

4. The propellant of claim l, there being ve unequallyangularly related slo-ts formed in said slots having a progressive angular relation of 25 degrees, 50 degrees, 75 degrees, 100 degrees and 110 degrees respectively.

5. An outward burning lgranulation for a jet assisted take-olf unit having. constant evolution of gas pressure and consisting of a propellent grain of cylindrical configuration having a predetermined outer diameter, there :being an axial perforation coextensive with the length ,of said grain, said grain having a length greater than `the web defined by the radial distance between the outer surface of said grain and the periphery of said perforastion, a .restrictive Vcoating covering one end of said grain `and the curved surface thereof, there being a plurality yof slots opening through the uncoated end of said grain and co-extending radially between said perforation and :the outer curved surface of said grain, said slots having .a depth approximately equal -to said web.

6. 'Ihe propellant of claim 5, there being a total of six slots formed Vin said grain, said slots being equally yangularly related.

7. The propellant of claim 5, there being a tot-a1 of `live slots, said s lot s being unequally angularly related.

8. The propellant of claim 5, there being a total of ive slots, said slots being unequally angularly related, said slots having a progressive angular relation of 25 degrees, 50 degrees, 75 degrees, 100 degrees and 110 degrees respectively.

9. An outward burnin-g granulation for a jet assisted take-off unit having constant evolution of gas and consisting `of a propellant grain of cylindrical configuration having an external diameter of 8.5 inches, there being an axial perforation coextensive with the length of said grain `having a diameter of v0.5 inch, said grain having a length of 16.5 inches, a restrictive `coating covering the curved surface of said grain and one end, there being a plurality of unequally angularly related slots co-extending radially between said perforation and the external surface of said grain and opening through the Auncoated end thereof,

said slots havin-g a depth of 4.1 inches, said slots also' having a progressive angular relation 0f 25 degrees, 50 degrees, degrees, 100 degrees and 110 degrees respectively.

10. An outward burning granulation for a jet assisted take-off unit having constant evolution of gas pressure consisting of, a cylindrical propellant having a length approximately twice the outer diameter of said grain, there being an axial perforation coextensive with the length of said grain, the radial burning distance between the outer surface of said grain and said perforation defining the web of said grain, a restrictive coating covering the curved surface of said grain and one end, there being a plurality of radial slots co-extending between said perforation and the outer surface of said grain and opening through the uncoated end thereof, said slots having a depth approximately equal to said web.

11. The grain of claim l0, said slots being equally angularly related.

12. The grain of claim 10, there being five -unequally angularly related slots.

13. The grain of claim 10, there being ve unequally angularly related slots, said slots having a progressive angular relation of 25 degrees, 50 degrees, 75 degrees,` degrees, and 110 degrees.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Iuly 27, 1895 

1. AN OUTWARD BURNING NEUTRAL GRANULATION FOR CAST PROPELLANTS CONSISTING OF A SOLID CYLINDRICAL GRAIN HAVING TOP AND BOTTOM SURFACES, A SIDE WALL AND A LENGTH GREATER THAN THE RADIUS OF SAID GRAIN, SAID GRAIN ALSO HAVING A LONGITUDINAL AXIS OF SYMMETRY AND AN AXIAL PERFORATION COEXTENSIVE WITH SAID DEPTH, A RESTRICTIVE COATING COVERING SAID SIDE WALL AND SAID BOTTOM SURFACE, THERE BEING A PLURALITY OF ANGULARLY RELATED SLOTS COEXTENDING RADIALLY OUTWARDLY BETWEEN SAID PERFORATION AND THE OUTER CURVED SURFACE OF SAID GRAIN, SAID SLOTS HAVING AN AXIAL DIMENSION SMALLER THAN SAID LENGTH AND EQUAL APPROXIMATELY TO SAID RADIUS, SAID SLOTS OPENING THROUGH SAID TOP SURFACE. 